The present application claims priority to Japanese Patent Application No. 2001-218684 filed in the Japanese Patent Office on Jul. 18, 2001. The contents of that application are incorporated herein by reference in their entirety.
1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine, a printer, or other similar image forming apparatus, and more particularly to an image forming apparatus that forms images using an intermediate transfer element including an elastic layer.
2. Discussion of the Background
In a known image forming apparatus, to form a toner image on a transfer material such as a transfer sheet, a latent image is first formed on a surface of a photoreceptor serving as a latent image carrier. Subsequently, the latent image on the photoreceptor is developed with toner, and then a toner image is transferred onto the transfer material. The transferred toner image is fixed onto the transfer material by heat and pressure in a fixing device, and thereby a print image is obtained.
In the case of a full-color image forming apparatus, four color toner images, such as black (BK), yellow (Y), magenta (M), cyan (C) toner images, formed on a photoreceptor are transferred onto an intermediate transfer element by each color by a primary transfer device (transfer from the photoreceptor to the intermediate transfer element may be hereinafter referred to as a xe2x80x9cprimary transferxe2x80x9d). The transferred color toner images are superimposed on each other on the intermediate transfer element. Subsequently, the superimposed color toner image is collectively transferred onto a transfer material by a secondary transfer device (transfer from the intermediate transfer element to the transfer material may be hereinafter referred to as a xe2x80x9csecondary transferxe2x80x9d).
The color toner image transferred onto the transfer material is fixed by a fixing device, and thereby a full-color image is obtained. As compared with an image forming apparatus in which a toner image is directly transferred onto a transfer material from a photoreceptor, the above-described full-color image forming apparatus using the intermediate transfer element has advantages in improving a defective image due to deviation of the position of color toner images at the time of superimposing each other, and an inferior transfer due to the difference in characteristics of transfer materials, etc. Therefore, such an image forming apparatus using an intermediate transfer element has been widely used.
As an example of an image forming apparatus using an intermediate transfer element, there is a so-called tandem type image forming apparatus including a plurality of photoreceptors and an intermediate transfer element. Specifically, the tandem type image forming apparatus includes a plurality of photoreceptors arranged in a line along a moving direction of an intermediate transfer element, and developing devices that respectively develop latent images on the photoreceptors with toner of respective colors. The respective color toner images formed on the photoreceptors are sequentially transferred onto the intermediate transfer element and superimposed on each other.
In the tandem type image forming apparatus, a space is required for arranging a plurality of photoreceptors in a line. To save space and reduce costs, the tandem type image forming apparatus often employs a belt-shaped intermediate transfer element. As compared to a drum-shaped intermediate transfer element, the belt-shaped intermediate transfer element has advantages in numerous layouts of devices in the image forming apparatus and in reducing the size and costs of the apparatus.
Further, the tandem type image forming apparatus includes a secondary transfer device that transfers a superimposed color toner image formed on the intermediate transfer element onto a transfer material. Specifically, the secondary transfer device includes a secondary transfer bias roller formed from, for example, a conductive elastic roller at a secondary transfer nip part formed between the intermediate transfer element and the secondary transfer device. The superimposed color toner image on the intermediate transfer element is transferred onto the transfer material by press-contacting the transfer material against the intermediate transfer element by the secondary transfer bias roller and by applying a secondary transfer bias to the secondary transfer bias roller.
Recently, in a monochromatic image, there are increasing demands for a high quality image in which fine thin lines are reproduced. In a color toner image formed by superimposing a plurality of color toner images on each other, there are also increasing demands for a high quality image that can reproduce further fine thin lines and a sharp image. To fulfill the demands, it has been proposed to use a developer including toner particles of small diameter. When using toner particles of small diameter, the particle diameter of magnetic carrier in a developer also needs to be small so as to charge the toner particle properly. However, as the particle diameter of magnetic carrier becomes smaller, a magnetic attraction force of the magnetic carrier per piece decreases. As a result, the magnetic carrier tends to attach to a photoreceptor by a bias applied between the photoreceptor and a developing device.
Especially, in the case of using a belt-shaped intermediate transfer element, magnetic carrier attached onto a halftone toner image portion having an intermediate image density tends to cause a so-called xe2x80x9cwhite spotxe2x80x9d when a toner image is transferred from an intermediate transfer element onto a transfer material (i.e., a secondary transfer). The xe2x80x9cwhite spotxe2x80x9d means a condition in which a toner image is partially omitted at around magnetic carrier on a transferred toner image on a transfer material.
There are increasing demands for forming full-color toner images on various kinds of transfer materials, e.g., thin and soft Japanese paper, a transfer material having concave/convex portions. When using such a transfer material having an uneven surface, a space is likely to be formed between a toner image and the transfer material at the time of transfer, thereby making occurrence of white spot conspicuous.
According to one aspect of the present invention, an image forming apparatus includes at least one latent image carrier configured to carry an electrostatic latent image, at least one developing device configured to develop the electrostatic latent image with developer to form a toner image on the at least one latent image carrier and an intermediate transfer element including an elastic layer and configured to carry the toner image transferred from the at least one latent image carrier. The image forming apparatus further includes a primary transfer device configured to transfer the toner image on the at least one latent image carrier onto the intermediate transfer element and a secondary transfer device configured to transfer the toner image carried by the intermediate transfer element onto a transfer material. The developer includes toner and magnetic carrier, and a weight average particle diameter of the magnetic carrier is in a range of 10 xcexcm to 80 xcexcm.